On 2017-07-07 13:40, Chris Murphy wrote:
On Fri, Jul 7, 2017 at 10:59 AM, Andrei Borzenkov <arvidj...@gmail.com> wrote:
07.07.2017 19:42, Chris Murphy пишет:
I'm digging through piles of list emails and not really finding an
answer to this. Maybe it's Friday and I'm just confused...
[root@f26s ~]# btrfs device ready /dev/sda1
[root@f26s ~]# echo $?
0
[root@f26s ~]# btrfs device ready /dev/mapper/vg-1
[root@f26s ~]# echo $?
0
/dev/sda1 is a single device btrfs and it is present, 'btrfs fi show' finds it.
/dev/mapper/vg-1 is one member of a two device Btrfs volume, the
missing device is /dev/mapper/vg-2 and 'btrfs fi show' sees 1 but not
2 and says a device is missing.
And yet the ready command completes and the exit code is the same, and
I just don't understand the purpose of this command. The man page says
"wait" but I don't see any waiting, so at the very least we probably
need to come up with a more descriptive man page description.
Here on Ubuntu 16.04.2 man page says
ready <device>
Check device to see if it has all of it’s devices in cache
for mounting.
4.11 man page says for 'btrfs device ready'
Wait until all devices of a multiple-device filesystem are scanned and
registered within the kernel module.
Not sure when it was updated.
Nothing about "wait".
btrfs device ready is only useful during startup. The only check it does
- whether kernel knows about all devices or not. It is really intended
to be used from udev rule to delay attempt to mount filesystem until all
devices have been seen. And it is only useful with systemd as it is the
only program that actually pays attention to it.
OK that make sense. I'm just not used to the convention where a user
space tool does literally nothing for the user, and also has a man
page entry as if it is for users. And I get that developers are users
too, it just seems like an odd convention but whatever.
TBH, it's not even entirely useful during startup. Even if all the
devices are supposedly there, the filesystem may still not mount because
of any number of other issues, the inherent TOCTOU race not
withstanding, and even if all the devices aren't there, the filesystem
may still mount depending on what options you pass.
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